President Trump's tariff turmoil, U.K. Prime Minister Theresa May's bungled Brexit negotiations, French President Emmanuel Macron's tax fiasco, etc. are all causing a slowdown in productive global investment and economic growth. The recent decline in the stock markets can be largely attributed to policy uncertainty. Mr. Trump is not to blame for the European mess, but his erratic trade actions and other unpredictable comments have contributed to the drop. He was on solid ground when he took much of the credit for the stock market rise, but now he needs to take some of the blame for the fall.